1. Field of the Invention
The present invention relates to method for manufacturing a GaN-based compound semiconductor light emitting device. A blue light emitting diode (LED) or a blue laser diode (LD) has a heterostructure of an InGaN layer and an GaN-based layer.
2. Description of the Related Art
Blue LEDs are used in full color displays, sign panels, and so on. Blue LDs are used in the next generation optical disk equipment. These devices, blue LEDs and LDs, are in great demand today.
One method for manufacturing typical GaN-based blue LED is as follows.
FIG. 1 shows a cross sectional view of the typical GaN-based blue LED and FIG. 9 is a timing chart describing the treatment temperature for growing the layers. In these growing steps, the metal organic chemical vapor deposition (MOCVD) is applied.
As shown in FIG. 9, the sapphire substrate 1 is thermally cleaned in a hydrogen gas flow at 1100.degree. C. (ST1), and the temperature is then brought down to 520.degree. C. in the same gas flow (ST2). After changing the hydrogen gas in ST1 to a carrier gas having mixture of nitrogen and hydrogen in a ratio of 1:3, at ammonia gas (NH.sub.3), and tri-methyl gallium (TMG), the GaN buffer layer 2 is formed at the thickness of 50 nm. The ammonia gas (NH.sub.3) is a source of group V of elements and the tri-methyl gallium (TMG) is a source of group III of elements. The ammonia gas is present the entire time in the steps of growing compound layers of the group V of elements, even in the steps during the suspension of growing.
After increasing the temperature to 1100.degree. C. an n-type GaN cladding layer 3 doped with Si and having a thickness of 4 .mu.m, is grown in the same carrier gas with the TMG and SiH.sub.4 (ST3). After growing the n-type GaN cladding layer 3, the temperature is decreased to 800.degree. C. With the temperature at 800.degree. C. stably, tri-methyl indium (TMI), TMG, SiH.sub.4 and di-methyl zinc (DMZn) in the carrier gas (N.sub.2 and H.sub.2 at the ratio of 1:1) grow an InGaN emitting layer 4 doped with Si and Zn to a thickness of 0.2 .mu.m on the n-type GaN cladding layer 3 (ST4). The TMI and the TMG produce the InGaN emitting layer 4 layer, and the SiH.sub.4 and DMZn provide the dopants Si and Zn. In this specification the ratio of N.sub.2 and H.sub.2 as 1:1 is also expressed as "inert gas rich" or "nitrogen rich".
After growing the InGaN emitting layer 4, the ratio of N.sub.2 and H.sub.2 in the carrier gas is moved to 1:3 (a hydrogen rich condition) and the temperature is brought up to 1100.degree. C. (ST55). At 1100.degree. C. with the hydrogen rich carrier gas, a p-type AlGaN layer 5 doped with Mg and having a thickness of 0.2 .mu.m is grown on the InGaN emitting layer 4 from a source gas having tri-methyl aluminum (TMA), TMG, and cyclopentadiphenyl magnesium (Cp.sub.2 Mg) added to the carrier gas. The Cp.sub.2 Mg is the source of the dopant Mg. The p-type GaN layer 6 having a thickness of 0.3 .mu.m, is grown in the gas containing TMG and Cp.sub.2 Mg (ST6). After the p-type GaN layer 6 is deposited, temperature in the deposition chamber decreases to room temperature in the same carrier gas. Thus, the GaN-based light emitter device is obtained.
When the p-type AlGaN layer 5 is grown on the InGaN emitting layer 4 in the above conventional process, the temperature and the ratio of H.sub.2 in the carrier gas must be increased after stopping the growth of the p-type InGaN emitting layer 4 (ST55) because the GaN grows at a temperature 200.degree. C. higher than the InN does. Thus, in order to grow the p-type AlGaN layer 5, the InGaN emitting layer 4 is exposed in the high temperature through increasing the temperature step (ST55), namely from 800 to 1100.degree. C. The InGaN emitting layer must hold fine in the high temperature atmosphere (about 800-1100.degree. C.) to grow the AlGaN layer on later.
The hydrogen rich carrier gas is applied in ST55 because it is easier to refine the hydrogen gas than the nitrogen gas. However, the InGaN emitting layer 4 is degraded or destroyed when the temperature increases from 800 to 1100.degree. C. under the hydrogen rich condition before growing the p-type GaN layer 6. It is difficult to maintain the quality of the InGaN emitting layer 4, so the light emitting efficiency and the reliance of the device deteriorate.